Vibration generation is a common means by which products are tested in their development and manufacturing stages. Most products will encounter some form of environmental vibration throughout their lifecycle; vibration testing is used to ensure product integrity in anticipation of these vibrations that may be present, for example, during transportation and in-service use. Any given product will likely be subject to a variety of vibration environments. If a device is mounted in an automobile, for example, then it will have to withstand vibration from driving on various road and terrain surfaces. It is rarely possible to test products in their in-situ environments, so these environments must be simulated with mechanical test systems. A common method of this simulation is with a Random Vibration Test System, which generates a random vibration with a frequency content tailored to match the expected frequency content of the anticipated real-world environment.
A Random Vibration Test System uses a shaker, which is a mechanical actuator that produces physical forces that are transmitted to a device under test. The shaker is controlled by a vibration controller, which generates an electrical signal that drives the shaker. The controller receives input from sensors that allow it to measure the physical response of the system. The controller uses this feedback to adjust its output so that the physical response meets a pre-defined profile.
The already well-known means by which a shaker generates these forces are varied and not described in this document. However, a limitation that affects all shakers is their displacement capacity, which is the total distance that their moving element can travel. At low frequencies, a much greater displacement is required to achieve a given acceleration amplitude than at high frequencies. The displacement demand increases as the inverse square of the frequency as the frequency decreases for a fixed acceleration.
A shaker system running a random vibration test needs to achieve a certain target, which is usually characterized by an acceleration spectrum that is measured in power spectral density units (PSD). In other words, the target of vibration is usually a curve defined as acceleration vs. frequency. The shaker system must generate this acceleration within its physical limitations.
Limited by the nature of its electrical-mechanical structure, the displacement of a shaker must always remain within a certain range. It becomes more difficult, and adds cost and complexity, to design and build a shaker with a larger displacement capability. For example, while a shaker with 5 mm displacement capacity costs a few thousand dollars, a shaker with 50 mm displacement capacity can cost a few hundred thousand dollars.
Due to the reasons above, if a vibration test can be conducted with a smaller displacement while achieving the same target acceleration PSD, it is a huge advantage. Consequently, controlling or minimizing the displacement of a random control signal will be exceedingly useful.